Process and apparatus for joining objects together

ABSTRACT

A process for joining objects, such as wooden boards, by means of a cement adapted to be hardened in an electric high-frequency field between a pair of electrodes, wherein the section of the objects situated between the electrodes is sealed off by means of a suitable layer of resilient material pressed against the surfaces of the objects to prevent steam and ionized gases from escaping from the objects to the surrounding space, thereby permitting a substantial increase of the electric energy applied without risk of spark discharge between the electrodes. The invention also provides a device for performing this process, comprising a pair of electrodes provided with a layer of such resilient material.

This invention relates to a process for joining objects together bymeans of a cement adapted to be hardened through the action of anelectric high-frequency field between a pair of electrodes applied topreferably opposite faces of the joined objects. The invention isparticularly, though not exclusively, concerned with the joining ofwooden boards end to end by means of a socalled finger joint, i.e. ajoint in which the ends of the boards to be joined are formed withalternate projections and recesses engaging each other finger-like. Theinvention will, therefore, hereinafter be described with a special viewto this utilisation thereof, however without being limited thereto.

When two boards are to be cemented together end to end, or side by side,they are placed with their joint between opposed electrodes to which ahigh frequency source supplies an amount of electric energy capable ofproducing such an amount of heat in the joint that the cement becomeshardened. The higher the level of the energy supplied is, the sooner thecement is hardened. On the other hand, the amount of energy supplied tothe electrodes must not be so high as to result in a spark dischargebetween the electrodes, which might be detrimental both to the wood andto the cement. For this reason, it has heretofore not been possible toreduce the hardening time of the cement to less than about 20 seconds.This value applies, for example, to a much used cement of the resorcinoltype.

It has been found that a primary cause of discharges between theelectrodes is the generation of steam and ionised gases in consequenceof the heat generated in the cement and in the wood under the action ofthe high-frequency field. Both the cement and the wood contain a certainamount of water which is, thus, caused to vaporise, thereby making thesurrounding air better conductive and, consequently, increasing the riskof discharges.

The invention aims to provide a process of the kind referred tohereinbefore which will avoid this inconvenience and which, accordingly,allows a much higher energy level to be used, so that the hardening timeof the cement is substantially reduced.

To this effect, the invention provides a process of the kind referredto, in which an intermediate layer of a steam-tight, resilientelectrically insulating material is applied to at least those faces ofthe objects to which the electrodes are applied, the intermediate layerbeing pressed firmly against these faces, preferably by means of theelectrodes. When these intermediate layers are pressed against thesurfaces of the objects to be joined, they adapt themselves to the formof the surfaces and seal all pores and other openings, including thejoint, towards the exterior. Accordingly, the steam generated under theaction of the heating of the objects and of the cement cannot penetrateinto the surrounding air to increase the conductivity thereof. Inpractice, it has been found that the level of the energy suppliedthrough the electrodes may be increased to such an extent that thehardening time of the cement is reduced to about 3 seconds, or less.

The steam-tight resilient intermediate layer may consist of any suitablematerial, such as rubber or other elastomer substances. Its thicknessdepends, on the one hand, of the resiliency of the material and, on theother hand, of the surface properties of the objects to be joined. Ifthe latter have a relatively rough surface, for example, theintermediate layer should be correspondingly thick and resilient inorder to be capable of being pressed sealingly against the surfaces,while inversely objects having a relatively smooth surface, such asplaned boards, will only require a correspondingly thinner and lessresilient intermediant layer.

Preferably, the intermediate layer has such dimensions and is soarranged as to extend in all directions beyond the contour of theelectrodes, whereby the potential discharge way becomes longer, and atthe same time any steam issuing from the objects outside the facescovered by the intermediate layer is kept remote from the areas of highfield intensity. Equally, also those faces of the objects which do notface the electrodes may be covered with a layer of steam-tight,resilient, electrically insulating material over a length correspondingat least to the extension of the electrodes in the direction of thefaces in question. This measure is particularly recommendable when theelectrodes are large enough to be used with all current dimensions ofobjects to be joined and, accordingly, in the case of objects of certaindimensions, such as boards having a relatively small width, will extendbeyond the objects in one or more directions. In this case, the measuredescribed will prevent steam and gas from issuing from the edge faces ofthe boards into the space between the protruding portions of theelectrodes.

The invention also provides a device for performing the describedprocess, and of the type having two or more electrodes adapted to beapplied to faces of the objects to be joined, wherein the faces of theelectrodes adapted to be applied to the objects are coated with a layerof steam-tight, resilient, electrically insulating material. Thus theintermediate layer of the invention constitutes part of the electrodesand is thus automatically applied to the objects to be joined when theelectrodes are pressed against the same by pneumatic or hydraulic means,for example. These means should, of course, be capable of exerting apressure which is greater than that with which the contained steam inthe objects strives to remove the electrodes from the surfaces of theobjects.

In a preferred form of the device intended for cementing transversejoints between oblong objects having a substantially rectangularcross-section, and having the electrodes arranged in parallel planesopposite each other to receive the objects between them, especially adevice for joining wooden boards end to end, a pair of opposed slidesare arranged for sliding movement towards and away from each otherbetween the planes of the electrodes in a direction parallel with theseplanes, the opposed ends of the slides being provided with a coatingconsisting of a steam-tight, resilient, electrically insulatingmaterial, the coating having a dimension in a direction at right anglesto the planes of the electrodes approximately equal to the distancebetween the electrodes when applied to the objects, and a dimension in adirection parallel with the planes of the electrodes and with thelongitudinal direction of the objects substantially equal to thedimension of the electrode coatings in this direction.

Reference will now be had to the accompanying drawings, in which:

FIG. 1 is a diagrammatical cross section of the essential parts of adevice in accordance with the invention,

FIG. 2 is a plan view of the same, and

FIGS. 3 and 4 are large scale representations of the area surrounded bya circle in FIG. 1, showing the parts immediately before pressure isexerted to urge the upper electrode against the objects to be joined,and after the pressing down of the upper electrode, respectively.

The device illustrated in the drawings is intended for joining woodenboards end to end in a socalled finger joint. The end faces of theboards are, as shown in FIG. 2, formed with a plurality of substantiallywedge-shaped notches with corresponding intermediate projectionsengaging the notches of the adjoining board, a suitable cement beingpreviously applied to the interengaging faces. The device comprises astationary lower electrode 2 mounted in a frame (not shown) and servingas a support for the adjoining ends of the boards, and a verticallyadjustable upper electrode 3 which by some suitable means, such as ahydraulic or pneumatic cylinder, can be urged against the upper faces ofthe boards 1. Both electrodes have relatively large dimensions in thetransverse direction of the boards, so as to be capable of use withvarious dimensions of boards. In the example shown, they extend arelatively great distance beyond the lateral edges of the boards, andlikewise their dimensions in the longitudinal direction of the boardsare substantially greater than the depths of the wedge-shaped notches ofthe end faces of the boards. The electrodes 2, 3 are connected to a HF,VHF, UHF or micro-wave generator (not shown) having a suitable poweroutput. Each electrode is, on the face thereof opposite the otherelectrode, coated with a layer 4 of a steam-tight, resilient,electrically insulating material, such as rubber or soft plastic, thislayer extending in all directions a certain distance beyond the contourof the electrode.

A pair of opposed slides 5 are mounted in the frame (not shown) forsliding movement towards and away from each other in such a manner thatthey can be introduced between the electrodes 2 and 3 to engage thelateral faces of the boards 1, as shown in FIG. 1. These slides which,like the electrode 3, are connected with a suitable prime mover, areprovided with a steam-tight, resilient, electrically insulating coating6 on their ends adapted to engage the boards. These coatings extend adistance in the longitudinal direction of the boards substantially equalto the dimension of the coatings 4 in this direction, while theirheights (at right angles to the planes of the electrodes) areapproximately equal to the thickness of the boards, or preferablysomewhat smaller, as shown in FIG. 3.

In this latter case, the electrode coatings 4, when pressed against theupper and lower faces of the boards, will be deformed around the sharpedges of the board to meet the slide coatings 6, as shown in FIG. 4. Inthis manner, the faces of contact between the coatings 4 and 6, whichconstitute a potential leak for steam from the boards, are kept at acertain distance from the sharp edges of the boards, through which thesteam would have a particular tendency to escape. Furthermore, thisdimensioning of the slide coatings 6 enable the slides to be used withboards of various thicknesses within a certain range. In thisconnection, the slides 5 should be mounted with a certain verticalclearance in order to allow them to adjust themselves between theelectrodes when the latter are pressed against the boards.

The manner of operation of the device described is as follows:

The boards to be cemented together are placed with their joint on thelower electrode 1, as shown in FIG. 2, and are at the same time urgedlongitudinally against each other by suitable means (not shown) whichare well-known in the art. The slides 5 are caused to engage the lateralfaces of the boards with a moderate pressure, and the upper electrode islowered against the upper faces of the boards, see FIG. 3. Now fullpressure is applied both to the slides and to the upper electrode, sothat the entire section of the boards situated between the coatings 4and 6 on both sides of the joint is enclosed in a steam-tight mannerbetween the coatings. Subsequently, the electrodes are connected to thehigh-frequency generator for some seconds, until the cement has becomehardened. During this hardening, the coatings 4, 6 prevent the steamgenerated from forcing its way out in the transverse directions of theboards, while at the same time the wood on both sides of the joint willoffer a sufficient resistance to the leaking of steam from the joint inthe longitudinal direction of the boards, at least for the short periodof time required for hardening the cement. Accordingly, the hardeningmay be effected at a high-frequency power level which could heretoforenot be utilised.

The invention is not limited to the example described hereinbefore andshown in the drawings. Thus, the process and the device in accordancewith the invention may also be used for other operations than that ofcementing boards together end to end, e.g. for cementing them togetherside by side or face to face, and with other forms of the joint, or forjoining other materials than wood.

What I claim is:
 1. A process for joining oblong objects having arectangular cross-section together end to end by means of a cementadapted to be hardened through the action of an electric high-frequencyfield, said process comprising the steps of placing oblong objects endto end with their joint containing cement between a pair of opposedelectrode plates adapted to generate a high-frequency field and with twoof their faces parallel to said electrode plates, two other faces ofsaid objects being at right angles thereto, applying a layer of asteam-tight, resilient, electrically insulating material to those areasof said faces parallel to and at right angles to said electrode platessituated within the space comprised between said electrode plates,pressing said layers firmly against said areas to prevent steam andgases from escaping therefrom, and supplying high-frequency electricenergy to said electrodes to harden said cement.
 2. A process as inclaim 1, wherein said layers have such dimensions and are so arranged asto extend in the longitudinal direction of said oblong objects beyondthe contours of said electrode plates.
 3. A device for joining oblongobjects of rectangular cross-section together end to end by means of acement adapted to be hardened through the action of an electrichigh-frequency field, comprising a pair of opposed, parallel normallyspaced electrode plates adapted to be forcibly applied to oppositeparallel faces of said objects, said electrode plates being coated ontheir faces adapted to engage said opposite parallel faces of saidobjects with a layer of steam-tight, resilient, electrically insulatingmaterial, and a pair of opposed slides arranged for sliding movementtowards and away from each other between said electrode plates in adirection parallel to said plates, said slides having opposed endsprovided with a coating consisting of a steam-tight, resilient,electrically insulating material and adapted to engage another pair ofopposed parallel faces of said oblong objects substantially over thewidth of said faces and along the length of the portions thereofdisposed between said electrodes.